In optical transport network WDM-signals [Wavelength Division Multiplex] are transmitted via optical fibers between a plurality of network nodes. A WDM-signal is composed of a plurality of optical channel signals, each channel signal is transmitted in an allocated channel with a corresponding wavelength via an optical path between the network nodes. Specifically in long-haul networks adjustments of channel signal power levels is required to ensure that certain target powers like amplifier input and/or output powers are reached at certain points such as nodes or a receiver.
The power measurements can be done at these places using external devices. Alternatively specific methods like pilot tones modulated onto the optical signals allow it to measure the received power levels, but they require complex arrangements.